Define __STDC_{LIMIT,CONSTANT,FORMAT}_MACROS in CXXFLAGS.
The problem is that the gnulib interception of <stdint.h> and <inttypes.h>
does not really work because we implement those internally with
<sys/inttypes.h> and <sys/stdint.h> and those internal headers are used
by other internal headers *before* they get a chance to be intercepted
(where the __STDC_ macros are defined).

Another way to fix this is to move the inclusion of the other headers
in <stdint.h> and <inttypes.h> outside multiple inclusion protection.

(christos)